As is known, there is a need to measure force and/or torque in numerous fields of application. To this end, for example, so-called six-axis load cells are available, as described for example by G. Mastinu, M. Gobbi and G. Previati in “A new six-axis load cell. Part I: design”, Experimental Mechanics 51: 373-388 (2011), or in patent WO2005/015146. Further examples of six-axis load cells, also known as load cells with six degrees of freedom, are described by S. M. Declercq, D. R. Lazor and D. L. Brown in “A smart 6-DOF load cell development”, Society for Experimental Mechanics (SEM), International Modal Analysis Conference XX, and by N. G. Tsagarakis, G. Metta, et al. in “iCub: the design and realization of an open humanoid platform for cognitive and neuroscience research”, Advanced Robotics, vol. 21, no. 10, pages 1151-1175 (2007).
In general, a six-axis load cell, which will henceforth be referred to as a load cell for brevity, is an electronic system designed to provide the measurements of three components of a force acting on the electronic system, these three components being measured with respect to a reference system of the load cell. In addition, the load cell is designed to provide the measurements of three components of torque acting on the load cell, these three components also being measured with respect to the reference system of the load cell.
In detail, the load cell has three radial arms, which are made in one piece and arranged such that pairs of radial arms form angles of 120°. Each radial arm extends from a central portion, made in one piece with the radial arms. In addition, the load cell comprises a frame, which encircles the radial arms.
In greater detail, each radial arm has a first end, which is connected to the central portion, and a second end, opposite to the first end and constrained to the frame. Furthermore, one or more sensors are mounted on each radial arm that are capable of detecting the deformation to which the associated radial arm is subjected following application of the aforesaid force and torque.
In particular, there are known load cells in which each radial arm substantially has the shape of a parallelepiped and in which a corresponding strain gauge is arranged on each face of the parallelepiped. Considering a radial arm and the four corresponding strain gauges, two of the four strain gauges, arranged on opposite faces of the radial arm, are electrically connected to a first pair of fixed resistors, so as to form a first Wheatstone bridge, of the so-called half-bridge type. Furthermore, the other two strain gauges are connected to a second pair of fixed resistors, so as to form a second Wheatstone bridge, this also of the so-called half-bridge type.
The strain gauges are electrically connected to a processing unit. In addition, each strain gauge provides an electrical signal indicative of a corresponding local stress.
Based on the signals supplied by the strain gauges and the (known) shape of the radial arms, the processing unit is capable of determining a corresponding local vector, formed by three pairs of local forces. Each pair of local forces includes a first and a second local force, which are orthogonal to each other and act on a corresponding radial arm. In particular, given a radial arm, the corresponding first and second local forces are the constraining reactions to which the radial arm is subjected to where it is constrained to the frame.
Based on the local vector, the processing unit is also capable of determining the aforesaid three components of the force acting on the load cell, as well as the aforesaid three components of the torque acting on the load cell, this force and torque acting on the geometric centre of the load cell. The reference system with respect to which this force and torque are calculated usually coincides with the reference system of the load cell's frame.
That having been said, in numerous fields of application there is a need for measuring the forces that are exerted when gripping an object. For example, there is a need for measuring the forces exerted by a human being or a robot when handling an object.
For example, the article by W. D. Memberg and P. E. Crago, “Instrumented objects for quantitative evaluation of hand grasp”, Journal of Rehabilitation Research and Development 34(1), pages 82-90, 1997 and the article by G. Kurillo, M. Gregoric, et al., “Grip force tracking system for assessment and rehabilitation of hand function”, Technology and Health Care 13(3), pages 137-149, 2005, describe electronic devices designed to measure the forces applied during a hand grasp.
Patent application WO2011/096367 describes a floor reaction force measuring device, which is constrained beneath the sole of a user's foot. The device comprises a first unit, suitable for being constrained close to the heel and a second unit, suitable for being constrained close to the big toe; both the first and the second units comprise three reaction force sensors, which are arranged in a planar manner.
Thus, referring to gripping forces to indicate the forces generated when an object is handled by a user, whether a human being or a mechanical device, the electronic devices of known type are characterized by the capacity to measure gripping forces quite accurately. However, these electronic devices have low flexibility of use, as each one of them is designed according to a corresponding grip mode; in other words, each electronic device has a shape that is predefined according to the expected use, i.e. depending on the manner in which the user is expected to handle the device. Possible variations with respect to the expected uses therefore entail profound changes to the electronic devices, and in particular to the respective electronic circuitry.